Process For Reducing Gate Blush and Mold Design For Same

ABSTRACT

A system and process for injection molding polymer articles is described. The system and process are designed to reduce gate blush. In one embodiment, an injection molding device injects a molten polymer composition into a mold cavity adjacent to an interior curved surface on the mold. The flow of the polymer material is parallel to a line that is tangent to the curved surface.

RELATED APPLICATIONS

The present application is based upon and claims priority to U.S.Provisional Patent Application Ser. No. 62/818,975, having a filing dateof Mar. 15, 2019, which is incorporated herein by reference.

BACKGROUND

In the relatively recent past, there has been an increasing desire toreplace metallic parts with parts made from polymer materials,particularly high performance thermoplastic polymers. The need extendsto almost a limitless variety of different applications and fields. Forinstance, parts made from thermoplastic polymers are used in consumerappliance products, industrial processes, in all sorts of transportationvehicles, and the like. For example, there is an increasing demand toreplace metal parts in the automotive field with parts made fromthermoplastic polymers. The parts made from the thermoplastic polymers,for instance, may be used on the exterior of the vehicle or on theinterior of the vehicle.

When used in automotive applications, for instance, the parts made fromthermoplastic polymers typically are required to have an aestheticappearance in order to further increase the appeal of the vehicle. Thus,in many applications, after being molded, the parts are then painted orotherwise decorated in order to color coordinate with the environment inwhich they are used. Unfortunately, painting the plastic partsrepresents a substantial expense in the production of the part. Havingto paint the part also increases production time and can significantlyadd capital expense to the facilities in which the parts are produced.

In view of the above, those skilled in the art have attempted to addcoloring agents to thermoplastic polymers in order to forego having topaint the molded parts. In one embodiment, for instance, metallicpigments are added to thermoplastic compounds to provide molded partshaving a metallic appearance. During injection molding of polymerarticles, however, various surface defects can appear during productionof the part. For example, certain part geometries, mold layouts, thelocation of gates, and molding conditions can cause various drawbacksincluding increased gloss loss on weathering and visual defectsaffecting part aesthetics. One particular reoccurring problem is knownas “gate blush” which results in irregular dark patterns or a dull ordiscolored area near the gate location of an injection mold. Anotherreoccurring problem is the formation of flow lines in the finished partformed at the meeting of two flow fronts when the mold configurationincludes more than one gate. These flow lines are sometimes referred toin the art as “knit-lines”.

Knit-lines can be eliminated in many applications by having a moldconfiguration that only includes a single gate or point of entry for themolten thermoplastic material into the mold. Attempts to eliminate gateblush, on the other hand, have generally been unsuccessful in manydifferent molding configurations. U.S. Patent Publication No.2013/0123424, which is incorporated herein by reference, for instance,discloses a method to reduce gate blush on a molded article by designingan injection mold runner that contains one or more overflow passages.The overflow passages are to divert polymer flow temporarily duringinjection which has been found to significantly reduce the occurrence ofgate blush. Further or alternative configurations for reducing oreliminating gate blush, however, are still desired.

SUMMARY

The present disclosure is generally directed to a molding process andsystem for injection molding articles that reduces or substantiallyeliminates gate blush. More particularly, the present disclosure isdirected to an injection molding system where gate blush is reduced bydesigning a gate for the flow of a polymer material that connects with amold cavity at a point that is tangent to a curved edge of the mold.

In one embodiment, for instance, the present disclosure is directed to aprocess for molding a polymer material. The process includes feeding amolten polymer material through a gate leading into a mold cavity. Themold cavity includes an interior curved surface. The gate is positionedsuch that a flow of the molten polymer material exits the gate at alocation that is tangent to the interior curved surface. The processfurther includes the steps of filling the mold cavity with the polymermaterial to form a molded article and removing the molded article fromthe mold.

The interior curved surface of the mold cavity, for instance, can followan arc that extends greater than about 50°, such as greater than about80°, such as greater than about 90°, such as greater than about 100° andgenerally less than about 300°. The arc defined by the interior curvedsurface, for instance, can generally follow a circle, e.g. a circle,ellipse, or oval. The exit of the gate can form an angle with the arcdefined by two radii of the circle. The angle can be greater than about30° and less than about 90°. For instance, the arc can have a first endand a second and opposite end. The angle can be formed on the circlehaving a vertex at the center of the circle and can extend between thefirst end on the arc and the point where the flow of polymer material istangent to the arc. This angle, for instance, can be from about 35° toabout 65°, such as from about 40° to about 50°, such as from about 42°to about 48°.

In addition to introducing the flow of polymer into the mold cavityalong an interior curved surface, flow of the polymer material can alsobe spread out in a fan-like shape when exiting the gate. For instance,the fan-like flow shape can have a width that is at least five timesgreater than a thickness of the polymer flow.

The polymer material deposited into the mold cavity can comprise anysuitable thermoplastic polymer. In one embodiment, for instance, thepolymer material may comprise a polyoxymethylene polymer or a polyamidepolymer. In one embodiment, the thermoplastic polymer can contain ametallic pigment, such as an aluminum pigment. In one embodiment, themetallic particles can have a plate-like shape. The plate-like shapedparticles, for instance, can have an aspect ratio of greater than about4:1, such as greater than about 10:1 and generally less than about 40:1.

The present disclosure is also directed to a system for injectionmolding a polymer article from a thermoplastic material. The systemincludes a mold that defines a mold cavity. The mold cavity includes aninterior curved surface. The system further includes a compression zonefor heating and compressing thermoplastic material and conveying themolten thermoplastic material into at least one flow path that leads toa gate. The gate is positioned so as to exit into the mold cavity. Moreparticularly, the gate is positioned such that flow of a molten polymermaterial through the gate exits the gate at a location that is tangentto the interior curved surface of the mold cavity. In one embodiment,the gate has a fan-like shape.

Other features and aspects of the present disclosure are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present disclosure is set forthmore particularly in the remainder of the specification, includingreference to the accompanying figures, in which:

FIG. 1 is a cutaway perspective view of one embodiment of an injectionmolding configuration in accordance with the present disclosure;

FIG. 2 is a geometric diagram illustrating the relationship between flowof a polymer and a curved surface on a mold in accordance with thepresent disclosure;

FIG. 3 is a cross-section of the molding configuration illustrated in

FIG. 1;

FIG. 4 is a further cross-section of the molding configurationillustrated in FIG. 1; and

FIG. 5 is a side view with cutaway portions of an injection moldingsystem in accordance with the present disclosure.

Repeat use of reference characters in the present specification anddrawings is intended to represent the same or analogous features orelements of the present invention.

DETAILED DESCRIPTION

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentdisclosure.

The present disclosure is generally directed to a system for producingmolded articles and to a process for molding articles while reducing oreliminating gate blush. In general, the system of the present disclosureincludes a mold defining a mold cavity in fluid communication with aninjection device. The injection device is configured to heat and injectmolten thermoplastic materials into the mold. Inbetween the mold and theinjection device is a flow path. The flow path ends in a gate that ispositioned adjacent to the mold cavity. In accordance with the presentdisclosure, a molten polymer material exiting the gate enters the moldcavity tangent to a curved portion on the interior surface of the mold.

More particularly, the present disclosure is directed to an injectionmold gate that dispenses a molten flow of polymer into a mold cavityalong a line that is tangent to a curved surface on the interior of themold. For example, the flow of polymer can be tangent to a curvedsurface on the interior of the mold at any point on the curved surfaceextending from greater than 1° to generally less than about 89°. Byhaving the gate dispense the polymer material into the mold cavity at apoint that is generally tangent to a curved surface on the mold cavity,gate blush can be significantly reduced and in many cases eliminated,even when the polymer material contains a metallic pigment or otherdispersed pigment.

Gate blush is a surface imperfection that can appear as a discolorationin the gate area of an injection-molded article. The gate area is theportion on the appearance surface of the product formed adjacent to thegate where the thermoplastic polymer is injected into the mold. The gatearea, for instance, can be circular and have a diameter of less thanabout 5 mm², such as less than about 4 mm², such as less than about 3mm², such as less than about 2 mm², such as less than about 1 mm². Thecenter of the gate area is concentric with the position of the gate. Asthe thermoplastic polymer is injection molded through the gate, gateblush can form and represent a series of curves located in the gatearea.

Gate blush can be defined in different ways. In one embodiment, forinstance, gate blush comprises a helical pattern of melt fractures that,in one embodiment, are visible to the human eye. In other embodiments,the helical patterns are visible when examined through a scanningelectron microscope at ×100 magnification and at an angle of 45°. In oneembodiment, at least some of the melt fractures have a length of atleast 20 microns, such as at least 20%, such as at least 40%, such as atleast 60%.

Gate blush can also be determined through a surface imperfectionmonitoring technique known as the BORG spectrophotometric technique,which is described in U.S. Pat. No. 6,868,371, which is incorporatedherein by reference. In this technique, a spatially resolvedspectrophotometer is used to measure surface imperfections in moldedplastic parts. The measurements from one or more of these sample plasticparts are then provided to a computerized device, which appropriatelyfilters the data and calculates overall data shape, average peak andvalley shift, and a quality number indicative of data slopes. In thismethod, a sample molded part is mounted on a sample holder that isoptionally mounted on motorized translation stages, so as to scan acrossthe feature of interest on the sample surface. A computerized deviceautomates the motion system and data collection, and transforms the rawdata into color coordinates. This data is processed by taking intoconsideration the instrument calibration data obtained beforehand, andallows the user to optimize the signal to noise by allowing adjustmentof parameters. The instrument then calculates the difference in thecolor index between the lightest and darkest points across a streakedregion.

Surface imperfections are typically detected by measurement of areduction in the color index (hereinafter sometimes referred to as “L”)which corresponds physically to the imperfection (e.g., the gate blush).

Using the above technique, a color index can be measured at a surfaceimperfection such as in the gate area of a molded product. The colorindex can also be measured away from the imperfection. A percentdifference in color shift can then be calculated from the twomeasurements.

In accordance with the present disclosure, minimized surfaceimperfections results in small differences in the color index measuredat the gate area and on the remainder of the appearance surface. Forinstance, in one embodiment, the color index in the gate area can varyby no more than 20%, such as by no more than 10%, such as no more than5% in the gate area versus the color index over the remainder of theappearance surface.

One method for using the BORG spectrophotometric technique is describedin U.S. Pat. No. 6,825,266, which is also incorporated herein byreference.

Referring to FIG. 5, one embodiment of an injection molding system thatmay be made in accordance with the present disclosure is shown. Asillustrated, the system includes an injection device 10 that isconfigured to melt or soften a polymer material and to inject the moltenpolymer material into a mold cavity 12. A polymer composition can be fedto the injection device 10 in any suitable manner. In the embodimentillustrated in FIG. 5, for instance, the system includes a hopper 14 forreceiving a polymer composition and supplying the composition to theinjection device 10.

The injection device 10 includes a barrel 16 which contains a heater anda conveying device. The conveying device may comprise, for instance, oneor more feed screws 18. For instance, in the embodiment illustrated inFIG. 5, the barrel 16 includes a single feed screw 18. In otherembodiments, however, the barrel 16 may contain dual feed screws. Thefeed screw 18 is rotated by a motor 20. The motor 20, for instance, maycomprise an electric motor that is connected to the feed screw 18 by oneor more belts or chains.

When a polymer composition is added to the hopper 14, the polymercomposition is heated within the barrel 16 into a molten state. The feedscrew 18 conveys the molten polymer material from the barrel 16 into aflow path 22. The flow path 22 can communicate with the injection device10 by an injection point. From the flow path 22, the polymer material isinjected into a mold cavity of the mold 12 via a gate 24 for producingmolded articles.

In the past, problems were experienced at the point of entry of thepolymer material into the mold cavity. In particular, visual defectsformed where the polymer material entered the mold cavity. These defectsare referred to as gate blush. In many instances, the gate blush canappear as an irregular dark pattern or as a web-like imperfection thatsubstantially decreases the aesthetic appeal of the part and can lead tothe part being rejected by manufacturers.

Referring to FIGS. 1-4, one embodiment of a mold configuration inaccordance with the present disclosure for reducing or eliminating gateblush is shown. Referring to FIG. 1, for instance, a flow path 22 isshown that is designed to receive the flow of a molten polymer materialfrom an injection molding system such as the one illustrated in FIG. 5.The flow path 22 is in fluid communication with a gate 24. The gate 24directs the flow of the molten polymer material into the cavity of amold 12.

As shown in FIG. 1, the mold 12 includes a curved portion 30 thatdefines an interior curved surface within the mold cavity. In accordancewith the present disclosure, the gate 24 is positioned such that a flowof the molten polymer material exits the gate at a location that istangent to the interior curved surface defined by the curved portion 30.

Referring to FIG. 2, a diagram is presented that illustrates therelationship between the curved portion 30 on the mold 12 and thedirection of flow of the polymer material. As shown, the curved portion30 includes an interior curved surface that generally follows thecircumference of a circle 34. It should be understood that the interiorcurved surface of the mold does not necessarily have to have a perfectlycircular shape. FIG. 2 is provided for purposes of explanation only.

A line 32 is illustrated in FIG. 2 that is tangent to the curved portion30. In accordance with the present disclosure, the gate 24 dispenses themolten polymer material in a direction that is parallel with the line 32on the interior of the mold 12. In general, the molten polymer materialcan flow into the mold cavity at any point on the curved portion 30 thatis tangent to the curved portion. For example, as shown in FIG. 2, thecurved portion 30 defines an arc 36. The arc has a first end 38 and asecond end 40. In the embodiment illustrated in FIG. 2, the arc 36defined by the curved portion 30 extends 90° on the circumference of thecircle 34. In general, the arc defined by the curved portion 30 canextend at least 50°, such as at least 60°, such as at least 70°, such asat least 80°, such as at least 90°, such as at least 100°, such as atleast 110°, such as at least 120°, such as at least 130°, such as atleast 140°, such as at least 150°, such as at least 160°, such as atleast 170°, such as at least 180°, such as at least 190°, such as atleast 200° and generally less than about 330°, such as less than about300°, such as less than about 250°.

As shown in FIG. 2, the point at which the line 32 or the polymer flowis tangent to the arc 36 can be defined by an angle θ. The angle θ isdefined by two radii of the circle 34. One side of the angle extendsfrom the center of the circle 34 to the first end of the arc 36. Thesecond side of the angle extends from the center of the circle 34 to thepoint at which the line 32 is tangent to the arc 36. In accordance withthe present disclosure, the angle θ can be adjusted as needed forreducing or eliminating gate blush. In general, the angle θ is greaterthan about 1° and less than about 89°. The angle θ for any particularmold configuration may depend upon various factors including the size ofthe mold cavity, the polymer material flowing into the mold, the size ofthe arc 36 defined by the interior curved surface 30, the flow rate ofthe polymer material, and various other factors. In one embodiment, theflow of the polymer material enters the mold cavity at a point tangentto the arc 36 such that the resulting angle θ is greater than about 10°,such as greater than about 20°, such as greater than about 30°, such asgreater than about 40°. The angle θ is generally less than about 80°,such as less than about 70°, such as less than about 60°, such as lessthan about 50°. In one embodiment, the angle θ is from about 35° toabout 65°, such as from about 40° to about 50°, such as from about 42°to about 48°. In the embodiment illustrated in FIG. 2, for instance, theangle θ is 45°.

Referring to FIGS. 3 and 4, a cross-sectional view of the moldconfiguration illustrated in FIG. 1 is shown. The mold 12 is in fluidcommunication with the gate 24. The mold 12 includes the curved portion30 that defines an interior curved surface. The gate 24 directs the flowof a molten polymer material such that the polymer material enters themold cavity of the mold 12 at a point tangent to the interior curvedsurface defined by the curved portion 30. As shown in FIG. 4, a circle34 can be drawn that best follows the interior curved surface of thecurved portion 30. The flow of polymer material enters the mold cavityat a point that is tangent to the interior curved surface. The pointthat is tangent on the interior curved surface forms an angle θ on thecircle 34. This angle can be anywhere from greater than 1° to less than89°, such as greater than about 30° to less than about 60°.

In one embodiment, the mold 12 includes only a single gate 24. As shownin FIG. 1, in one embodiment, the gate 24 can have a fan-like shape. Asthe molten polymer material flows into the gate 24 from the flow path22, for instance, the molten polymer material is spread out as it entersthe mold cavity of the mold 12. The fan-like shape of the gate 24, forinstance, can have a width W that is much greater than the height H. Thewidth W, for instance, can be at least 3 times greater, such as at least5 times greater, such as at least 7 times greater, such as at least 9times greater than the height H. The width W is generally less thanabout 50 times greater than the height H.

As also shown in FIG. 1, the gate 24 can generally have a graduallydecreasing height H from the flow path 22 to the exit of the gate at theentrance to the mold 12. The gradually decreasing height is also shownin FIG. 4.

In general, the system and process of the present disclosure areadaptable to processing any suitable thermoplastic polymer in a moldingprocess, particularly in an injection molding process. In oneembodiment, the polymer composition used in the process of the presentdisclosure includes a thermoplastic polymer combined with a coloringagent for producing molded articles that are ready for use upon exitingthe mold cavity. In particular, the coloring agent is present in orderto provide a molded article having aesthetic appeal and to forego thenecessity of having to paint the molded article or otherwise decoratethe molded article.

Thermoplastic polymers that may be processed according to the presentdisclosure include polyacetal polymers, polyamide polymers, polyarylenesulfide polymers such as polyphenylene sulfide polymers, polyolefinpolymers including polyethylenes and polypropylenes, polycarbonatepolymers, polyester polymers including PCT polymers, and the like.

The coloring agent combined with the thermoplastic polymer may comprisea pigment, a dye, metallic particles, and the like. Pigment particlescan include, for instance, barium sulfate particles, titanium dioxideparticles, calcium carbonate particles, and the like. In anotherembodiment, the coloring agent may comprise mica flakes alone or incombination with dyes, such as organic dyes.

In one particular embodiment, the polymer composition comprises apolyoxymethylene or polyamide resin combined with a coloring agent,particularly metal flakes. The polymer resin may comprise a homopolymeror a copolymer and can include end caps. The polyoxymethylenehomopolymers may be obtained by polymerizing formaldehyde or trioxane,which can be initiated cationically or anionically. The homopolymers cancontain primarily oxymethylene units in the polymer chain.Polyoxymethylene copolymers, on the other hand, may contain oxyalkyleneunits along side oxymethylene units. The oxyalkylene units may contain,for instance, from about 2 to about 8 carbon units and may be linear orbranched. In one embodiment, the homopolymer or copolymer can havehydroxy end groups that have been chemically stabilized to resistdegradation by esterification or by etherification.

Polyoxymethylene copolymers can contain alongside the —CH₂O— repeatunits, up to 50 mol %, such as from 0.1 to 20 mol %, and in particularfrom 0.5 to 10 mol %, of repeat units of the following formula

where R¹ to R⁴, independently of one another, are a hydrogen atom, aC₁-C₄-alkyl group, or a halo-substituted alkyl group having from 1 to 4carbon atoms, and R⁵ is —CH₂—, —O—CH₂—, or a C₁-C₄-alkyl- orC₁-C₄-haloalkyl-substituted methylene group, or a correspondingoxymethylene group, and n is from 0 to 3.

These groups may advantageously be introduced into the copolymers by thering-opening of cyclic ethers. Preferred cyclic ethers are those of theformula

where R¹ to R⁵ and n are as defined above.

Cyclic ethers which may be mentioned as examples are ethylene oxide,propylene 1,2-oxide, butylene 1,2-oxide, butylene 1,3-oxide,1,3-dioxane, 1,3-dioxolane, and 1,3-dioxepan, and comonomers which maybe mentioned as examples are linear oligo- or polyformals, such aspolydioxolane or polydioxepan.

Use is also made of oxymethyleneterpolymers, for example those preparedby reacting trioxane with one of the abovementioned cyclic ethers andwith a third monomer, preferably a bifunctional compound of the formula

where Z is a chemical bond, —O— or —ORO—(R═C₁-C₈-alkylene orC₂-C₈-cycloalkylene).

Preferred monomers of this type are ethylene diglycide, diglycidylether, and diethers composed of glycidyl units and formaldehyde,dioxane, or trioxane in a molar ratio of 2:1, and also diethers composedof 2 mol of glycidyl compound and 1 mol of an aliphatic diol having from2 to 8 carbon atoms, for example the diglycidyl ethers of ethyleneglycol, 1,4-butanediol, 1,3-butanediol, 1,3-cyclobutanediol,1,2-propanediol, or 1,4-cyclohexene diol, to mention just a fewexamples.

Polyacetal resins as defined herein can also include end capped resins.Such resins, for instance, can have pendant hydroxyl groups. Suchpolymers are described, for instance, in U.S. Pat. No. 5,043,398, whichis incorporated herein by reference.

In one embodiment, the polyacetal polymer may contain hemiformalterminal groups and/or formyl terminal groups. In particular, it isbelieved that the methods of the present disclosure can advantageouslysignificantly reduce formaldehyde emissions of polyacetal polymers, evenwhen the polymers contain hemiformal terminal groups and possibly formylterminal groups. For instance, in one embodiment, the polyacetal polymermay contain hem iformal terminal groups in amounts greater than 1.0mmol/kg, such as in amounts greater than 1.5 mmol/kg. In an alternativeembodiment, the polyacetal polymer may contain formyl terminal groups inamounts greater than 2 mmol/kg, such as in amounts greater than 2.5mmol/kg.

The processes used to form the polyoxymethylene polymers as describedabove can vary depending upon the particular application. A process,however, can be used which results in a polyacetal resin having arelatively low formaldehyde content. In this regard, in one embodiment,the polymer can be made via a solution hydrolysis process as may bedescribed in U.S. Patent Application Publication Number 2007/0027300and/or in United States Patent Application Number 2008/0242800, whichare both incorporated herein by reference. For instance, in oneembodiment, a polyoxymethylene polymer containing aliphatic orcycloaliphatic diol units can be degraded via solution hydrolysis byusing methanol and water with triolethylene.

Polyacetal resins or polyoxymethylenes that may be used in accordancewith the present disclosure generally have a melting point of greaterthan about 150 degrees C. The molecular weight of the polymer cangenerally range from about 2,000 to about 1,000,000, such as from about7,000 to about 150,000. The polymer can have a meltflow rate (MVR190-2.16) from about 0.3 to about 20 g/10 min, and particularly fromabout 2 to about 9 g/10 min (ISO 1133).

Various different types of polyamide polymers may also be used toproduce the molded articles. The polyamide polymer, for instance, may bean aliphatic polyamide, such as nylon 6 or nylon 66.

In general, the polymer composition can contain a thermoplastic polymerresin in an amount greater than about 60% by weight, such as in anamount greater than about 70% by weight, such as in an amount greaterthan about 80% by weight, such as in an amount greater than about 90% byweight. The polymer is generally present in an amount less than about95% by weight.

As described above, the polymer resin may be combined with a metallicpigment, such as metal flakes. The metal flakes can have a plate-likeshape. The particles can be polished or otherwise have a highreflectivity. In one embodiment, the plate-like particles can have anaspect ratio of greater than about 4:1, such as greater than about 8:1,such as from about 10:1 to about 50:1.

The plate-like particles can have a median diameter of greater thanabout 12 microns, such as greater than about 14 microns. For instance,the plate-like particles can have a median diameter of from about 15microns to about 650 microns. In one particular embodiment, theplate-like particles have a size of from about 15 microns to about 30microns.

The metallic pigment may be present in the polymer composition in anamount from about 0.01% to about 20% by weight, such as from about 0.1%to about 15% by weight. For example, the metallic pigment may be presentin the polymer composition in an amount from about 0.25% to about 10% byweight, such as from about 0.5% to about 5% by weight.

The metallic pigment may comprise any suitable metal, such as metals ofGroup I-B, III-A, IV, VI-B, and VIII of the Periodic Table. Physicalmixtures of these metallic pigments and alloys of these pigments mayalso be employed. In particular examples, the metallic pigment mayinclude aluminum, bronze, brass, chromium, copper, gold, iron,molybdenum, nickel, tin, titanium, zinc and the like. In one embodiment,two metallic pigments may be combined that have different averageparticle sizes.

In one embodiment, the metallic pigment may comprise an aluminum pigmentthat contains elemental aluminum. The aluminum pigment, for instance,can be very thin having a thickness of less than about 1 micron and canhave a median diameter as described above. In one particular embodiment,for instance, the aluminum pigment may have a median diameter of fromabout 12 microns to about 18 microns. The aluminum pigment can have apronounced flop, a high brilliance and be highly reflective.

In one embodiment, the aluminum pigment can contain greater than about80% by weight aluminum. The aluminum pigment can be present alone or incombination with other additives, such as a carrier. For instance, thealuminum pigment may be present in combination with a thermoplasticpolymer, such as a polyolefin, a purified medical white oil, or may bepresent with a solvent, such as di-isononyl-phtalate.

In addition to a coloring agent and a thermoplastic polymer, the polymercomposition may also contain various other components. For instance, inone embodiment, an ultraviolet light stabilizer may be present. Theultraviolet light stabilizer may comprise a benzophenone, abenzotriazole, or a benzoate. Particular examples of ultraviolet lightstabilizers include 2,4-dihydroxy benzophenone, 2-hydroxy-4-methoxybenzophenone, 2-(2′-hydroxy-3′,5′-di-t-butylphenyl)benzotriazole,2-(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotriazole,2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone,2-hydroxy-4-octoxybenzophenone, and 5,5′-methylenebis(2-hydroxy-4-methoxybenzophenone);2-(2′-hydroxyphenyl)benzotriazoles, e.g.2-(2′-hydroxy-5′-methylphenyl)benzotriazole,2-(2′-hydroxy-5′-t-octylphenyl)benzotriazole,2-(2′-hydroxy-3′,5′-di-t-butylphenyl)benzotriazole,2-(2′-hydroxy-3′,5′-di-t-butylphenyl)-5-chlorobenzotriazole,2-(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotriazole,2-(2′-hydroxy-3′,5′-dicumylphenyl)benzotriazole, and 2,2′-methylenebis(4-t-octyl-6-benzotriazolyl)phenol, phenylsalicylate, resorcinolmonobenzoate, 2,4-di-t-butylphenyl-3′,5′-di-t-butyl-4′-hydroxybenzoate,and hexadecyl-3,5-di-t-butyl-4-hydroxybenzoate; substituted oxanilides,e.g. 2-ethyl-2′-ethoxyoxanilide and 2-ethoxy-4′-dodecyloxanilide;cyanoacrylates, e.g. ethyl-α-cyano-β,β-diphenylacrylate andmethyl-2-cyano-3-methyl-3-(p-methoxyphenyl)acrylate, or mixturesthereof.

In one embodiment, the polymer composition may also include aformaldehyde scavenger, such as a nitrogen-containing compound. Thenitrogen scavenger, for instance, may comprise a guanamine compound,such as benzoguanamine.

In one embodiment, the composition may contain a nucleant. The nucleant,for instance, may increase crystallinity and may comprise anoxymethylene terpolymer. In one particular embodiment, for instance, thenucelant may comprise a terpolymer of butanediol diglycidyl ether,ethylene oxide, and trioxane. The nucleant can be present in thecomposition in an amount greater than about 0.05% by weight, such asgreater than about 0.1% by weight. The nucleant may also be present inthe composition in an amount less than about 2% by weight, such as in anamount less than about 1% by weight.

Still another additive that may be present in the composition is asterically hindered phenol compound, which may serve as an antioxidant.Examples of such compounds, which are available commercially, arepentaerythrityltetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (Irganox 1010,BASF), triethylene glycolbis[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate] (Irganox 245,BASF), 3,3′-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionohydrazide](Irganox MD 1024, BASF), hexamethylene glycolbis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (Irganox 259,BASF), and 3,5-di-tert-butyl-4-hydroxytoluene (Lowinox BHT, Chemtura).Preference is given to Irganox 1010 and especially Irganox 245. Theabove compounds may be present in the composition in an amount less thanabout 2% by weight, such as in an amount from about 0.01% to about 1% byweight.

Light stabilizers that may be present in addition to the ultravioletlight stabilizer in the composition include sterically hindered amines.Hindered amine light stabilizers that may be used include oligomericcompounds that are N-methylated. For instance, another example of ahindered amine light stabilizer comprises ADK STAB LA-63 lightstabilizer available from Adeka Palmarole. The light stabilizers, whenpresent, can be included in amounts greater than about 0.1% by weight,such as in amounts greater than about 0.5% by weight, but in an amountless than about 2% by weight, such as in an amount less than about 1% byweight.

Fillers that may be included in the composition include glass beads,wollastonite, loam, molybdenum disulfide or graphite, inorganic ororganic fibers such as glass fibers, carbon fibers or aramid fibers. Theglass fibers, for instance, may have a length of greater than about 3mm, such as from 5 to about 50 mm. The composition can further includethermoplastic or thermoset polymeric additives, or elastomers such aspolyethylene, polyurethane, polymethyl methacrylate, polybutadiene,polystyrene, or else graft copolymers whose core has been prepared bypolymerizing 1,3-butadiene, isoprene, n-butyl acrylate, ethylhexylacrylate, or mixtures of these, and whose shell has been prepared bypolymerizing styrene, acrylonitrile or (meth)acrylates.

In one embodiment, the composition may also contain one or morelubricants. The lubricant may comprise a polymer wax composition.Lubricants that may be included in the composition include, forinstance, N,N′-ethylene bisstearamide. In one embodiment, a polyethyleneglycol polymer (processing aid) may be present in the composition. Thepolyethylene glycol, for instance, may have a molecular weight of fromabout 1000 to about 5000, such as from about 3000 to about 4000. In oneembodiment, for instance, PEG-75 may be present. Lubricants cangenerally be present in the polymer composition in an amount from about0.01% to about 5% by weight. For instance, a lubricant can be present inan amount greater than about 0.1% by weight, such as in an amount fromabout 0.1 to about 1% by weight. The above polyethylene glycol polymercan also be present in an amount up to about 5% by weight. For instance,the polyethylene glycol polymer can be present in an amount from about0.1% to about 2% by weight, such as from about 0.5% to about 1% byweight.

In addition to the above components, the polymer composition may alsocontain an acid scavenger. The acid scavenger may comprise, forinstance, an alkaline earth metal salt. For instance, the acid scavengermay comprise a calcium salt, such as a calcium citrate. The acidscavenger may be present in an amount of from about 0.01% to about 1% byweight.

Any of the above additives can be added to the polymer composition aloneor combined with other additives. In general, each additive is presentin an amount less than about 5% by weight, such as in an amount lessthan about 2% by weight, such as in an amount less than about 1% byweight.

An almost limitless variety of polymer articles may be molded inaccordance with the present disclosure. Such articles may include knobs,door handles, automotive panels, interior automotive parts such asbezels, consumer appliance parts, and the like without limitation.

The present disclosure may be better understood with reference to thefollowing example.

Example

The following example was conducted in order to demonstrate some of thebenefits and advantages of the present disclosure.

A polyoxymethylene polymer was blended with an aluminum pigment and fedthrough a single screw extruder followed by injection molding into aplaque using different gate geometries. The first tested gate geometrywas similar to the gate geometry illustrated in FIG. 3, where thepolymer flow was fed into the mold parallel with a line that is tangentto a curved surface on the mold. A second gate geometry was tested inwhich the gate dispensed the polymer into the mold where the polymerflow was not tangent to a curved surface on the mold. In particular, thegate was moved to a horizontal position with respect to the moldinterior surface (0° as shown in FIG. 4). In the first tested gategeometry, the angle of gate entrance to the mold was 45°. The moldmachine and injection molding parameters were kept constant for bothgate configurations.

After molding, the plaques were visually inspected. The plaque madeaccording to the present disclosure had no visible gate blush. Thecomparative sample, however, had gate blush issues covering over half ofthe surface of the plaque.

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood that aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in such appended claims.

What is claimed:
 1. A process for molding a polymer material comprising:feeding a molten polymer material through a gate leading into a moldcavity, the mold cavity including an interior curved surface, andwherein the gate is positioned such that a flow of the molten polymermaterial exits the gate at a location that is tangent to the interiorcurved surface; filling the mold cavity with the polymer material toform a molded article; and removing the molded article from the mold. 2.A process as defined in claim 1, wherein the interior curved surfacefollows an arc that extends greater than 50°.
 3. A process as defined inclaim 2, wherein the arc extends greater than about 80° and generallyless than about 300°.
 4. A process as defined in claim 1, wherein an arcdefined by the interior curved surface generally follows a circle, andwherein the exit of the gate forms an angle with the arc defined by tworadii of the circle, the angle being greater than 30° and less than 90°.5. A process as defined in claim 4, wherein the arc defines a first endand a second and opposite end and wherein the angle is formed on thecircle between the first end of the arc and the point where the flow ofpolymer material is tangent to the arc.
 6. A process as defined in claim4, wherein the angle is from about 35° to about 65°.
 7. A process asdefined in claim 4, wherein an angle formed between two radii of thecircle that extend from a center of the circle to the first end of thearc and a center of the circle to the second end of the arc has an angleof greater than about 50° and generally less than about 300°.
 8. Aprocess as defined in claim 1, wherein a flow of the polymer material isspread out in a fan-like shape when exiting the gate.
 9. A process asdefined in claim 8, wherein the fan-like flow shape has a width that isat least five times greater than a thickness of the polymer flow.
 10. Aprocess as defined in claim 1, wherein the polymer material comprises apolyoxymethylene polymer or a polyamide polymer.
 11. A process asdefined in claim 1, wherein the polymer material contains a metallicpigment.
 12. A process as defined in claim 11, wherein the metallicpigment comprises aluminum particles.
 13. A process as defined in claim11, wherein the metallic pigment comprises metal particles having aplate-like shape, the metal particles having an aspect ratio of greaterthan about 4:1.
 14. A system for injection molding a polymer articlefrom a thermoplastic material comprising: a mold defining a mold cavity,the mold cavity including an interior curved surface; a compression zonefor heating and compressing a thermoplastic material; a flow path influid communication with the compression zone for receiving a flow of amolten polymer material; and a gate in fluid communication with thepolymer flow path, the gate including an exit that leads into the moldcavity, the gate being positioned such that a flow of molten polymermaterial exits the gate at a location that is tangent to the interiorcurved surface of the mold cavity.
 15. A system as defined in claim 14,wherein the interior curved surface follows an arc that extends greaterthan 50°.
 16. A system as defined in claim 15, wherein the arc extendsgreater than about 80° and generally less than about 300°.
 17. A systemas defined in claim 14, wherein an arc defined by the interior curvedsurface generally follows a circle, and wherein the exit of the gateforms an angle with the arc defined by two radii of the circle, theangle being greater than 30° and less than 90°.
 18. A system as definedin claim 17, wherein the arc defines a first end and a second andopposite end and wherein the angle is formed on the circle between thefirst end of the arc and the point where the flow of polymer material istangent to the arc.
 19. A system as defined in claim 17, wherein theangle is from about 35° to about 65°.
 20. A system as defined in claim17, wherein an angle formed between two radii of the circle that extendfrom a center of the circle to the first end of the arc and a center ofthe circle to the second end of the arc has an angle of greater thanabout 70° and generally less than about 300°.
 21. A system as defined inclaim 14, wherein the gate has a fan-like shape, the gate including awidth and a height and wherein the width is at least four times greaterthan the height at the exit of the gate.